An electrocardiogram ("ECG" or "EKG") is a graphic representation of the electrical activity generated by the heart during the cardiac cycle and is recorded from the body surface. Electrocardiography is described in Greenfield, J. C., Jr. "Electrocardiography," in Cecil Textbook of Medicine, Volume 1, 19th Ed., edited by Wyngaarden, J. et al., pp. 170-175, which pages are hereby incorporated herein by reference.
Vectorcardiography ("VCG") is a variant of electrocardiography ("ECG") dating back more than half a century. In 1946 Burger et al described the summation of electrocardiographic impulses into vectors in three orthogonal planes X, Y and Z. Heart-Vector And Leads. Br Heart J 1946;8:15714 163, which is incorporated herein by reference. Ten years later, Ernst Frank presented the lead system on which modem vectorcardiographic monitoring is based. Accurate, clinically practical system for spatial vectorcardiography. Circulation, 1956;13:737-744, which is incorporated herein by reference. In the early 1960's various authors described the use of vectorcardiograms in the diagnosis of acute myocardial infarction. Wolff et al, Vectorcardiographic diagnosis: a correlation with autopsy findings in 167 cases. Circulation 1961;23:861-880; Hoffman et al Vectorcardiographic residua of inferior infarction: seventy-eight cases studied with the Frank system. Circulation 1964;29:562-576; and Young et al The frontal plane vectorcardiogram in old inferior myocardial infarction: criteria for diagnosis and electrocardiographic correlation. Circulation 1968;37:604-623, each of which are incorporated herein by reference.
Continuous vectorcardiographic monitoring was first described in Hodges et al Continuous recording of the vectorcardiogram in acutely ill patients. Am Heart J 1974;88:593-595, which is incorporated herein by reference. This method was later used in several studies by Sederholm and co-workers who found a correlation between the evolution of QRS changes and the release of myoglobin and creatin kinase due to injury to the heart muscle. Similar relationships were found between the duration of ST vector magnitude changes and enzymatic release and chest pain.
Further development of continuous online VCG monitoring was made by Martin Riha at Ostra Hospital Goteborg in collaboration with the Calmers Institute of Technology and Ortivus Medical AB. This resulted in the MIDA system (Myocardial Infarction Dynamic Analysis, Ortivus Medical, Taby, Sweden) which was introduced for clinical use as a tool for continuous on-line vectorcardiographic monitoring of ischaemia in 1986.
An ECG signal is a reflection of the electrical activity of the heart. The ECG signal differs, depending upon the placement of electrodes on the body, but generally it can be understood as a representation of a vector moving within the heart and seen from different angles in three-dimensional space, for example, as shown in FIG. 1A. The electrical vector of each part of the electrical process is the sum of all the electrical charge movements at a given moment in time. The mean QRS vector is shown in FIG. 1B. The different parts of the electrical process are denoted with the letters P, Q, R, S, T and U (FIG. 2). The first part of the electrical process represents electrical activity in the atria and is called P-wave. The Q, R and S portions of the wave are generally considered to represent the depolarization of the ventricle muscle, while the T-wave may represent repolarization of the muscle and inversion of the T-wave can signal ischaemia of the heart.
The level of the line connecting the S and T waves is called the S-T segment and deviation of that level from the normal baseline (isoelectric line), for example, as shown in FIG. 3, is generally considered to represent ischemia or injury of the heart muscle. Generally, inversion of the T-wave and depression of the S-T segment (as shown, for example, by the waves in FIGS. 4A and 4B, respectively) may represent cardiac muscle ischemia and injury. The more the S-T segment deviates from the baseline, the more severe the ischemia is considered. However, as with all parts of the electrical process in the heart, the S-T level looks different from the different electrodes of the ECG, as the electrical vector's relative position (magnitude) is different with respect to each electrode. In order to obtain a consistent measure of the level of the S-T segment it is therefore better to use the length of the electrical vector during this part of the process.
A major drawback of ECG systems is that the heart is connected to the body in a manner which permits limited movement. Thus, the heart changes its position within the thorax when people change their posture. As a result, an ECG can only be interpreted accurately if the subject is still during the recording, usually lying in a supine position.
Systems have been developed to allow simultaneous measurement of ECG signals and other signals which reflect movement of different parts of the body. For example, systems, e.g., as shown in U.S. Pat. No. 4,817,628 and International App. Patent PCT/FI95/00425, use accelerometers to correlate the movement of the subject with his ECG. The aim of these systems is to study and monitor the subject's movement and to then correlate such movement with the anomalous ECG readings. These systems require analysis of an ECG to determine whether anomalous signals are the result of disease or movement of the subject.
U.S. Pat. No. 4,993,421 discloses a cardiac monitoring system that generates ECG signals, and that employs an accelerometer to detect the activities of the subject. The monitoring system correlates changes in the ECG with changes in the physical activities of the subject to enable a physician to determine how physical activities effect the subject's ECG. However, the device disclosed in this patent does not provide any mechanism for correcting normal changes in ECG based upon the position or posture of the subject.